1. Field of the Invention
The present invention is directed to shale shakers with vibrating screens, and, in one aspect, to an apparatus for releasably retaining the screens.
2. Description of Related Art
The prior art disclosed a wide variety of vibrating screens, devices which use them, shale shakers, and screens for shale shakers. In shale shakers which use a plurality of screens, problems arise relative to easily and efficiently securing and replacing the screens.
The need for solids control in drilling mud in hydrocarbon well drilling is well known in the prior art. Drilling mud, typically a mixture of clay, water and various additives, is pumped through a hollow drill string (pipe, drill collar, bit, etc.) down into a well and exits through holes in a drill bit. The mud picks up cuttings (rock bits) and other solids from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the solids-laden mud is introduced to a shale shaker, a device which typically has a series of screens arranged in tiered or flat disposition with respect to each other. The screens catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create viscosity and gel problems in the mud, as well as increasing wear in mud pumps and other mechanical equipment used for drilling. In some shale shakers, a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth. The frame of the vibrating screen is suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism, e.g. an unbalanced weight on a rotating shaft connected to the frame. Each screen may be vibrated by vibratory equipment to create a flow of trapped solids toward an end of the screen on a top surface of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
In certain prior art shale shakers having a plurality of screens, releasably securing and lower level. This is particularly true if wooden wedges are used to secure the screens.
U.S. Pat. No. 5,392,925 discloses the use of a pair of wedges with their inclined surfaces being in contact. The lower wedge rests on mounting bars. The upper wedge has a threaded bolt attached to it on one end. The other end of the bold extends through the side of the basket and is engaged by a nut. As the nut tightens, the upper wedge slides up the lower wedge exerting an upward force onto a lower surface of the screen frame to maintain the upper surface of the screen frame against a mounting surface of the basket.
U.S. Pat. No. 5,971,159 discloses a cam latch secured to an outside surface of a side wall of the basket. A latching bar extending through an opening in the side wall of the basket is used to secure the screen frame against a mounting bracket. A pivot pin rides in a cam slot to adjust the height of the latching bar to a thickness of the screen frame. As a handle is pivoted, latching bar pivots about the pivot pin and presses against the screen frame against the mounting bracket.
Accordingly, there is a need to enhance the efficiency and ease with which screens are replaced and secured.
In one aspect of the present invention, there is provided a shale shaker having at least a base, a shaker screen, a basket, and a screen retainer for releasably mounting the shaker screen to the basket. The basket is mounted movably with respect to and on the base. The screen retainer has at least a cam surface and a tensioning device. The tensioning device has at least a bearing surface in contact with the cam surface, a spring responsive to the bearing surface, and at least one screen contact surface for engaging the shaker screen when the spring is placed in compression by the exertion of force by the cam surface on the bearing surface.
In one embodiment, the basket has a side wall and a sleeve attached to the side wall with a hole extending substantially perpendicular to the side wall through the sleeve and side wall. The screen retainer has a rod, a device for axially rotating the rod, and a disk having a center, a perimeter and a perimeter surface. The rod extends through the hole and the rod has an outside portion and an inside portion. The outside portion is attached to device for axially rotating the rod. The inside portion of the rod is attached to the disk between the center and the perimeter of the disk and the perimeter surface is the cam surface.
The tensioning device is formed in part of a polymeric material. Further, the tensioning device has a recess, such as a hole or a saddle, capable of receiving the disk. The recess is defined by the bearing surface. The spring is an elongated spring embedded in the polymeric material and laying in a plane which is substantially parallel to the side wall when the tensioning device is in an installed position. The at least one screen contact surface is two screen contact surfaces extending below a lower surface of the tensioning device. The elongated spring is located between the hole or saddle and the screen contact surfaces. The two screen contact surfaces are spaced from each other to allow deflection of the lower surface and the elongated spring when the screen retainer is in a position for retaining the shaker screen.
Alternatively, the tensioning device has an upper portion, an elongated spring having first and second end portions and a mid portion, and two screen contact surfaces. The elongated spring is located between the upper portion and the two screen contact surfaces. One screen contact surface is located on the first end portion. The other screen contact surface is located on the second end portion. The upper portion is attached to the mid portion and has a recess, such as a hole or a saddle, capable of receiving the disk. The recess is defined by the bearing surface.
In another alternative, the tensioning device has a first portion having a recess, such as a hole or a saddle, capable of receiving the disk and a second portion formed of an elastomeric polymer. The second portion is attached to the first portion. The recess is defined by the bearing surface and has a screen contact portion. The second portion deforms in part when the tensioning device is retaining a shaker screen which results from compressing a part of the second portion (i.e., a part of the elastomeric polymer) between the perimeter surface and the shaker screen. Due to its elastomeric properties, the compressed part of the second portion acts as a spring under load wanting to return to its original state once the force exerted by the cam surface onto the bearing surface is released.